RFR: 8322503: Shenandoah: Clarify gc state usage

[William Kemper]

This is a follow up to https://github.com/openjdk/jdk/pull/17112.

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Commit messages:
 - Rename gc state methods for clarity, assert that only java threads use thread local copy of gc state

Changes: https://git.openjdk.org/jdk/pull/17163/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17163&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322503
  Stats: 19 lines in 5 files changed: 1 ins; 0 del; 18 mod
  Patch: https://git.openjdk.org/jdk/pull/17163.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17163/head:pull/17163

PR: https://git.openjdk.org/jdk/pull/17163